(I wouldn't have noticed the slope if the other poster hadn't mentioned rotation, so it is probably best left alone. That said and I'm not suggesting you do this; I'd try skewing the right side down a tiny bit, but then I'm not exactly a purist, either.)

At risk of over analyzing the image: I think the white shoulder stripe terminating higher on the right edge of the photo than the stripe from the left side of the road contributes more to the perceived tilt. Although the center line of the photo is aligned with the VP, there is simply more road on the right. You can't change the way the highway was built. Unfortunately, these types of photos invite scrutiny of the geometry. I wonder if moving your watermark to the right side would make the effect better or worse, though I'm sure you have more interesting things to do :)

Maybe it's a combination of several things. Maybe I wasn't perfectly centered, cloud-horizon distance, funny built road, slope, mountains on one side, less mountains on the other, and maybe a little setup error..and there we have it: a puzzling picture.